Patent GB-1319209-A (GB)
Title: Vehicle brake systems
IPC: B60T11/34
Filed: 1971-09-09
Abstract: 1319209 Differential braking; warning systems; valves GENERAL MOTORS CORP 9 Sept 1971 [17 Sept 1970 (2)] 42038/71 Headings F2E and F2F A vehicle hydraulic brake system with a dual master cylinder 24 and separate circuits for front disc brakes 20, 22 and rear drum brakes 30, 32 incorporates a valve assembly which includes a metering section 34 which prevents initial front brake pressure build up until the rear brake shoes contact the drums, a warning section 36 which acts to give warning of loss of pressure in either circuit, and a proportioner section 44 comprising a rear brake pressure proportioner and a proportioner over-ride arrangement to remove the action of the proportioner in the event of loss of pressure in the front circuit. The metering section 34 comprises a metering valve pin 54 having a smooth head 56 separated by a groove 58 from a knurled section 60, and the section 34 also includes a valve pin stop 48 with openings 50, and a metering valve 80 having an outer portion 82 and an axially extending annular lip section 88. A valve retainer and spring seat 90 presses against the metering valve 80 and a spring 92 acts against the seat 90 and a seat 94 on the diaphragm seal 64. A further spring 100 is held between the seat 94 and a seat 98 on the pin 54. When the master cylinder 24 is initially operated fluid passes through an inlet 14, a chamber 46, openings 50, a chamber 52 and the axially grooved openings formed by the knurled section 60 and into a chamber 78 from where it is applied via outlets 16, 18 to disc brakes 20, 22. The light initial pressure in chamber 78 also acts on the diaphragm seal 64 to move pin 54 to the left against spring 100 until a shoulder 102 of section 60 engages the inner annulus of the spring retainer 90, at which time head 56 engages lip 88 to block off further flow from chamber 52 to chamber 78. Flow to chamber 78 is resumed when increasing pressure in chamber 46, 52 acts to move the pin 54, the valve 80 and the spring retainer 90 leftwardly whereupon inlet pressure is admitted to the chamber 78 past the outer periphery of seat 90: this pressure acts on the diaphragm seal 64 to move pin 54, retainer 90 and valve 80 further to the left until a pin land 66 abuts a shoulder 106. Complete release of brake pressure is subsequently accomplished by lifting of the lip 88. The warning section 36 includes a switch assembly 38 operable to energize an indicator 118 such as a light, bell or a horn. The switch is closed by upward movement of a switch plunger 42 when a switch piston assembly 136 moves to right or left upon loss of pressure in either brake circuit. The assembly 136 includes a piston 138 whose left end is exposed to master cylinder front brake pressure in the chamber 46 and whose right end is formed with a recess bore 174 opening into a chamber 172 connected to a rear brake pressure inlet 26. The proportioner section 44 has a proportioning valve piston 192 received in a chamber 190 and extending into the chamber 172 and formed with a valve seat 218. The valve seat 218 is co-operable with a valve element 224 on a valve stem 220. A piston stop 204 formed with openings 206 is mounted at the end of chamber 190 adjacent a rear brake pressure outlet 28 and provides a stop for piston 192 and valve stem 220. Upon initial application of master cylinder rear brake pressure fluid passes through inlet 26 into chamber 172 and between valve element 224 and seat 218, openings 206 and outlet 28. As pressure builds up the piston 192 is moved leftwardly and valve 218, 224 closes to give pressure modulation. During brake release piston 192 is moved leftwardly to engage a stop 186, taking with it valve stem 220 which has a lost motion connection with piston 138 including a spring 176. Should front brake pressure fall, resulting in a need for full back pressure application, the piston 138 moves leftwardly and a grooved guide 178 on valve stem 220 is engaged by a tab 180 to move it leftwardly. The element 224 cannot now co-operate with seat 218 to give a proportioning action. The piston 192 slides on a surface 232 of a seal support member 186 and to give good alignment this surface 232 is formed with a soft metallic plating which quickly wears away in a stroke or a few; the plating is of thickness 0À0002 to 0À0004 inch, and is of tin, lead or cadmium. A bleed arrangement comprising a groove 228 and seal 230 is formed in the proportioner section.